Vehicle braking system

Abstract

A vehicle braking system comprising a controller responsive to wheel speed signals from at least two wheel speed sensors for sensing skid conditions of at least two wheels and for generating skid control instructions; a plurality of wheel brakes responsive to fluid pressure, hereinafter referred to as brake pressure, supplied thereto; a supply device, comprising a common supply valve, for supplying brake pressure to the brakes from a fluid pressure supply in accordance with a brake operating signal; and a skid controller controlled by the skid control instructions for controlling the brake pressure in a plurality of cycles each of which comprises a pressure reduction phase and a pressure increase phase, and a first comparitor to compare the co-efficient of friction of the lower friction surface with a predetermined threshold, and/or a second comparitor to compare the relative values of the co-efficients of friction of the surfaces with which at least two of the wheels are engaged and a device to operate the skid controller in a select low mode if the comparitors determine that a) the co-efficient of friction of the lower friction surface is above a predetermined absolute level, and/or b) the co-efficients of friction of the surfaces with which at least two of the wheels are engaged differ by less than a predetermined amount, and in a select high mode if the comparitors determine that a) the co-efficient of friction of a lower friction surface is at or below a predetermined absolute level, and/or b) the co-efficients of friction of the surfaces differ by at least the predetermined amount.

Having described the invention, what is claimed is: 
     
       1. A vehicle braking system comprising: (i) at least two wheel speed sensors which generate wheel speed signals indicative of the speed of rotation of the respective wheel;   (ii) a controller responsive to said wheel speed signals for sensing skid conditions of at least two wheels and for generating skid control signals;   (iii) a fluid pressure supply;   (iv) brake command means which when operated generate a brake operating signal;   (v) a plurality of wheel brakes responsive to fluid pressure supplied thereto;   (vi) a supply means, comprising a common supply valve, for supplying brake pressure to said brakes from said fluid pressure supply in accordance with said brake operating signal;   (vii) skid control means responsive to said skid control instructions for controlling said brake pressure in a plurality of cycles each of which comprises a pressure reduction phase and a pressure increase phase;   (viii) at least one comparison means selected from the group comprising: (a) first comparison means arranged to compare the co-efficient of friction of the lower friction surface with a predetermined threshold, and   (b) second comparison means arranged to compare the relative values of the co-efficients of friction of the surfaces with which at least two wheels are engaged; and     (ix) means to operate the skid control means in a select low mode if at least one condition is met from the group comprising: (a) the co-efficient of friction of the lower friction surface is above said predetermined threshold; and   (b) the co-efficients of friction of the surfaces with which at least two wheels are engaged differ by less than a predetermined amount, and in a select high mode if at least one condition is met from the group comprising:     (a) the co-efficient of friction of the lower friction surface is at or below said predetermined threshold, and   (b) the co-efficients of friction of the said surfaces differ by at least said predetermined amount.     
     
     
       2. A vehicle braking system according to claim 1 wherein the comparison means comprises said first and said second comparison means and means to operate the skid control means in a select low mode if said comparison means determines that at least one condition is met from the group comprising: a) the co-efficient of friction of the lower friction surface is above a predetermined absolute level, and   b) the co-efficients of friction of the surfaces with which at least two of the wheels are engaged differ by less than a predetermined amount, and a select high mode if said comparison means determines that at least one condition is met from the group comprising:   a) the co-efficient of friction of a lower friction surface is at or below a predetermined absolute level, and   b) the co-efficients of friction of said surfaces differ by at least said predetermined amount.   
     
     
       3. A vehicle braking system according to claim 1 comprising means to monitor which wheel first encounters a skid condition when the brake pressure is increased in a pressure increase phase subsequent to a pressure reduction or holding phase pursuant to the detection of the first skid condition, and means to monitor the co-efficient of friction of the higher friction surface and in a second or greater skid occurrence of said wheel which first encountered a skid condition. 
     
     
       4. A vehicle braking system according to claim 1 comprising means to monitor the co-efficient of friction of the higher friction surface wherein the second comparison means comprises means to control the brake pressure above a threshold for a limited time period, and the system comprising: i) means to cause the controller to adopt a select high mode in response to a skid condition of the wheel engaged with said higher friction surface being above a threshold during the limited time period,   ii) means to cause the controller to adopt a select low mode, in response to a skid condition of the wheel engaged with said higher friction surface being below the threshold during limited time period.   
     
     
       5. A vehicle braking system according to claim 4 comprising means to effect a pressure reduction phase until the wheels have recovered to within a predetermined speed of vehicle speed in response to detection of a skid condition of the wheel engaged with said higher friction surface during the limited time period so that the controller adopts a select low mode. 
     
     
       6. A vehicle braking system according to claim 1 wherein the first comparison means comprises means to compare the speed of the wheels with a threshold and to cause the controller to be in select low mode if the speed of the wheel engaged with the lower friction surface is (low friction wheel) at or above the threshold. 
     
     
       7. A vehicle braking system according to claim 6 wherein the first comparison means comprises means to compare the acceleration of the wheel with a threshold if the speed of the low friction wheel is below the threshold, and to cause the controller to be in a select low mode if the acceleration is at or above the threshold and to cause the controller to be in a select high mode if the acceleration is below the threshold. 
     
     
       8. A vehicle braking system according to claim 1 wherein said means to control the brake pressure during said limited time period is operable so as to a) decrease during the time period but at a lower rate than in the previous recovery phase, or   b) remain constant during the limited time period, or   c) increase during the time period.   
     
     
       9. A vehicle braking system according to claim 8 wherein the means to control the brake pressure is operable so that when the brake pressure is increased it is increased at the same rate, or at a faster rate, or at a slower rate than in the previous pressure increase phase. 
     
     
       10. A vehicle braking system according to claim 1 comprising means to hold the brake pressure, or to further reduce and then hold the brake pressure to allow recovery of said wheel previously engaged with the lower friction surface in response to the controller being in a select high mode, and, during a pressure reduction phase or a pressure holding phase, the controller detecting acceleration of the wheel previously engaged with the lower friction surface, as a result of the co-efficient of friction of the higher and lower friction surfaces converging. 
     
     
       11. A vehicle braking system according to claim 1 comprising means further to reduce the brake pressure and optionally to then hold the brake pressure, to allow full recovery of the other wheel engaged with the lower friction surface in response to the controller being in a select high mode, and, during a pressure reduction phase or a pressure holding phase, the controller detecting that acceleration of the wheel previously engaged with a higher friction surface is within a predetermined acceleration of the acceleration of the wheel previously engaged with the low friction surface and/or is below a predetermined level, as a result of the co-efficient of friction of the higher and lower surfaces converging. 
     
     
       12. A vehicle braking system according to claim 1 comprising means, operative when the controller is in a select high mode, to compare the speed of the wheel engaged with the lower friction surface (low friction wheel) with a threshold, and means to cause the controller to adopt select low mode if the speed is at or above the threshold. 
     
     
       13. A vehicle braking system according to claim 1 comprising means to compare the speed of the high friction wheel with a threshold, if the speed of the low friction wheel is below a threshold, and means to cause the controller to maintain select high mode if the speed of the high friction wheel is greater than or equal to the threshold and means to cause the controller to compare the acceleration of the high friction wheel with a threshold if the speed of the high friction wheel is less than the threshold. 
     
     
       14. A vehicle braking system according to claim 13 further comprising means to cause the controller to maintain select high mode if the acceleration of the high friction wheel is greater than or equal to the threshold and to adopt select low mode if the acceleration of the high friction wheel is less than the threshold. 
     
     
       15. A vehicle braking system according to claim 1 wherein the wheels are steered wheels and means are provided to limit the brake pressure after the wheel engaged with the lower friction surface has skidded. 
     
     
       16. A vehicle braking system according to claim 15 wherein said means to limit the brake pressure may end the pressure increase phase after a predetermined time and then hold the brake pressure at that pressure. 
     
     
       17. A vehicle braking system according to claim 1 wherein the second comparison means comprises means to compare the speed of the wheel engaged with the higher friction surface (high friction wheel) with a threshold during a test procedure in which the brake pressure is maintained above a threshold for a limited time period and means to cause the controller to maintain select low mode if the speed of the high friction wheel is less than said threshold and to cause the controller to adopt a select high mode if the speed of the high friction wheel is greater than or equal to the threshold. 
     
     
       18. A vehicle braking system comprising: (i) at least two wheel speed sensors which generate wheel speed signals indicative of the speed of rotation of the respective wheel;   (ii) a controller responsive to said wheel speed signals for sensing skid conditions of at least two wheels and for generating skid control signals;   (iii) a fluid pressure supply;   (iv) brake command means which when operated generate a brake operating signal;   (v) a plurality of wheel brakes responsive to fluid pressure supplied thereto;   (vi) a supply means, comprising a common supply valve for supplying brake pressure to said brakes from said fluid pressure supply in accordance with said brake operating signal;   (vii) skid control means responsive to said skid control instructions for controlling said brake pressure in a plurality of cycles each of which comprises a pressure reduction phase and a pressure increase phase;   (viii) comparison means arranged to compare the relative values of the co-efficient of friction of the surfaces with which at least two wheels are engaged and comprising means to control the brake pressure above a threshold for a limited time period;   (ix) means to monitor the co-efficient of friction of the higher friction surface;   (X) means to cause the controller to adopt a select high mode in response to the speed of the wheel engaged with the higher friction surface being above a threshold during the limited period; and   (xi) means to cause the controller to adopt a select low mode in response to the speed of the wheel engaged with said higher friction surface being below the threshold during the limited time period.   
     
     
       19. A vehicle braking system according to claim 18 wherein said means to control the brake pressure during said limited time period is operable so as to a) decrease during the time period but at a lower rate than in the previous recovery phase, or   b) remain constant during the limited time period, or   c) increase during the time period.